Field of the Invention
The invention relates to a method of determining very small capacitances of electrical components, which can be used in particular in the case of capacitive surface sensors, in particular in the case of fingerprint sensors.
Swedish Patent Document SE 448 408 discloses a finger print sensor with capacitors which are disposed in columns and rows. The finger print sensor has read lines and control lines.
U.S. Pat. No. 5,343,157 discloses a method and apparatus for measuring an unknown capacitance using a known reference capacitance. The known reference capacitance is pumped onto the capacitor to be measured until the capacitor to be measured is charged to a given voltage.
It is accordingly an object of the invention to provide a method of measuring capacitances which is suitable for measuring very small capacitances and which is in particular suitable for an image sensing, for example of fingerprints.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of determining capacitances, the method includes the steps of:
(a) providing a grid-shaped configuration of capacitors, the capacitors being formed by pairs of electrical conductor surfaces;
(b) dividing the electrical conductor surfaces into groups;
(c) electrically conductively connecting the electrical conductor surfaces of in each case one of the groups to one another by a respective read line;
(d) electrically conductively connecting the electrical conductor surfaces of in each case one of the groups separately from one another to a respective control line;
(e) applying, during a given time, an electric potential to the respective control line such that a potential difference is present between the respective control line and a plurality of read lines and charging, with the potential difference, the capacitors formed in each case by at least one of the electrical conductor surfaces connected to the respective control line and at least one of the electrical conductor surfaces connected to one of the read lines;
(f) carrying off a charge via each of the read lines separately to a respective collecting capacitor;
(g) switching off the electric potential applied to the respective control line and impressing, at low resistance, a respective potential onto a respective one of the read lines by using a circuit, the respective potential defining a charge state of the respective collecting capacitor with respect to a reference potential;
(h) repeating steps (e) to (g) until a given number of charge operations has taken place; and
(i) subsequently determining for each of the read lines one of a charge of the respective collecting capacitor and a potential difference of the respective collecting capacitor.
With the objects of the invention in view there is also provided, a method of determining capacitances, which includes the steps (a) to (g) as defined above and further includes the steps:
(h) repeating steps (e) to (g) until a potential difference which is greater than a given value is present at the respective collecting capacitor; and
(i) subsequently determining for each of the read lines a number of performed charge operations.
In other words, the method according to the invention determines very small capacitances of a grid-shaped configuration of capacitors, wherein:
(a) the capacitances are formed by pairs of electrical conductor surfaces;
(b) a division of the conductor surfaces into groups is carried out;
(c) the conductor surfaces of one group each are connected in an electrically conducting manner to one another by a read line;
(d) the conductor surfaces of one group each are connected in an electrically conducting manner separately from one another to one control line each;
(e) during a predetermined time, an electric potential is applied respectively to a control line in such a way that there occurs between the control line and a plurality of read lines a potential difference which charges the capacitors which are formed by in each case at least one conductor surface connected to the control line and in each case at least one conductor surface connected to one of these read lines;
(f) the charge is carried away overreach read line separately to in each case one collecting capacitor;
(g) the potential applied to the control line is switched off and a respective potential, which defines the charge state of the collecting capacitor concerned with respect to a reference potential, is impressed at low resistance onto the read line concerned through the use of a circuit;
(h) steps (e) to (g) are repeated until a predetermined number of charges has taken place or until a potential difference which is greater than a predetermined value is present at each collecting capacitor; and
(i) then the charge or potential difference of the collecting capacitor or the number of charges is determined for each read line.
According to another mode of the invention, step (g) is performed by using in each case a feedback operational amplifier; and between in each case one of the capacitors to be measured and the respective collecting capacitor, an output of the feedback operational amplifier is switched to the respective read line, at the latest at a point in time at which the electric potential applied to the respective control line is switched off.
According to yet another mode of the invention, a circuit including two p-channel MOSFETs and three n-channel MOSFETs is used as the operational amplifier, the source terminals of the p-channel MOSFETs and the drain terminal of the first n-channel MOSFET being connected to a terminal of a supply voltage, the drain terminals of the p-channel MOSFETs being connected to the drain terminals of one each of the second and third n-channel MOSFETs, the gate terminals of the p-channel MOSFETs being connected to one another and to the drain terminal of the third n-channel MOSFET, the source terminals of the second and third n-channel MOSFETs being connected to one another and via a current source to another terminal of the supply voltage, the gate terminal of the first n-channel MOSFET being connected to the drain terminal of the second n-channel MOSFET, the source terminal of the first n-channel MOSFET being connected to the gate terminal of the second n-channel MOSFET and via a further current source to the other terminal of the supply voltage and forming the output of the circuit, and the gate terminal of the third n-channel MOSFET forming the input of the circuit.
According to a further mode of the invention, a structured image surface including the grid-shaped configuration of capacitors is provided; and a capacitive image sensing is performed by using steps (a) to (i) defined above. In other words, the method according to the invention is preferably used for a capacitive image sensing with a structured conductor surface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining very small capacitances, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.